ISFET characteristics in CMOS and their application to weak inversion operation

Design and fabrication of ISFETs in an unmodified CMOS process is presented to identify the main factors modifying the intrinsic characteristics of the MOSFET from which it is made and derive a model for its operation and pH sensitivity in weak inversion. Specifically trapped charge in the passivation layer and passivation capacitance introduced due to the method of fabrication have been identified and measured in a commercial 0.25 μm CMOS process. Functional operation in weak inversion is shown, for sensor operation using extremely low currents and therefore low power, as well as enabling it to become an inherent part of the CMOS integrated circuit design process allowing creation of building blocks for biochemical VLSI systems.